1. Field of the Invention
The present invention relates to an interference method and an interferometer for testing the surface precision of a parabolic mirror.
2. Description of the Related Art
A parabolic mirror is a useful optical unit which can be used for the purpose of converging collimated light rays towards a single point within or out of the incident optical path on which they travel towards the unit without aberration, and for the purpose of collimating spherical light rays diverging from a single point and directing the collimated light rays within or out of the incident optical path. Parabolic mirrors are used, for example, as a converging mirror in laser apparatus, such as are used in nuclear fusion devices or for laser machining purposes. In general, the precision with which the surface of a parabolic mirror which has been ground or finished is tested by directing a beam of collimated light rays onto the parabolic mirror and by measuring the size of the focal point to which the collimated light rays converge. Then, the surface precision of the parabolic mirror is determined in terms of the focal point thus measured. The surface precision increases with the decrease of the focal point, which can be used as the figure of merit. However, this testing method measures, in fact, the capability of converging light rays rather than the precision with which a parabolic mirror surface has been shaped. For another example of a testing method, a knife edge is disposed at a point at which light rays converge, and the surface precision is determined in terms of the brightness and darkness of an image. This method, however, is qualitative rather than quantitative in nature.
Another testing method uses a computer to produce a hologram, but it has not been practically used because of the complexity of forming such a hologram.
Still another testing method uses a spherical wave which is closest to the parabolic surface of a mirror to be tested. The surface precision of the mirror is determined by measuring the positions of interference fringes resulting from interference between the spherical wave and one reflected from the mirror. The analysis of interference fringes, however, is very troublesome, also requiring much time and work.